Eye tracking is useful for some types of systems and devices. For example, a system may analyze an image of a human face in order to determine a gaze direction that indicates where the eyes are looking or for determining a position of the head. Tracking of the eyes may be utilized in various ways. In some systems, tracking of a gaze direction may be used to control a displayed image. For example, if it is determined from the gaze direction that a user is reading a portion of text at the edge of a display screen, the display system may automatically scroll the text so as to enable the user to continue reading the text without interruption or without manually manipulating controls. Similarly, a section of an image at which the user is looking may be automatically centered in the field of view. Display control by eye tracking may be especially useful in a handheld device with a relatively small screen, such as a smartphone, or when manual control is difficult or inadvisable.
Tracking an object may be made difficult by the varied appearance of the object in different situations. The appearance of the object may constantly change during tracking due to changes in apparent size due to changes in distance of the object from the tracking camera. Other properties of the object that may change during tracking may include real or apparent shape, perceived color (e.g. due to non-uniform coloring of the object as is changes orientation or due to changing lighting or illumination), and orientation. In the particular case of eyes, the appearance of eyes may be different from person to person. Differences in facial features from person to person or presence of glasses frames may also increase the difficulty of identifying or tracking eyes in an image of a face. A low quality image sensor may also contribute to difficulty in tracking.
Thus, accurately tracking an object in real time, especially of subtle eye movements, may impose severe demands on a camera or processor resources used in tracking the object.